Capacitor with tin-zinc electrodes

ABSTRACT

A capacitor having electrodes consisting essentially of 65-95 parts tin and 35-5 parts zinc.

This is a division of Application Ser. No. 725,958 filed Sept. 23, 1976 now U.S. Pat. No. 4,130,854.

Capacitors with dielectrics of barium titanate have been made with electrodes of fired in silver paint consisting essentially of a pigment of silver, a frit for adhering the silver to the ceramic, ingredients for promoting solderability and an inert or indifferent vehicle which is eliminated during firing. After painting the electrode patterns on the dielectric, firing in air at a temperature of about 1500 degrees F. (816 degrees C.) eliminates the vehicle and fuses the frit to the ceramic and to the silver thereby producing the finished electrodes.

This invention eliminates the silver, the solderability promoting ingredient and the frit and substitutes a non-noble metallic pigment of tin, nickel, zinc and produces electrodes which are the functional equivalent of the silver electrodes. The frit is not needed. The non-noble pigment has the necessary adherence. A small amount (2-15%) of frit may be added for additional adherence. No change is necessary in the processing heretofore used for producing silver electrodes.

FIGS. 1 and 2 represent uses of the metallization of the application.

In the drawing,

FIG. 1 is an edge view of a ceramic disc capacitor having metallizations providing the electrodes, and

FIG. 2 is a diagramatic view of a monolithic capacitor.

The disc capacitor 1 is an exemplar of devices in which metallizations may be used. The term "metallization" as used herein refers to a powder of pigment sized particles which may be dispersed in an inert liquid vehicle to form a "metallizing composition". The latter is useful to print desired electrode patterns on dielectric substrates which, upon firing, produce conductors. It comprises a dielectric 2 of barium titanate or the like, electrodes 3,4 fired on the dielectric and in capacity relation to each other through the dielectric and leads 5,6 soldered or otherwise mechanically and electrically bonded to the electrodes as indicated at 7,8. The leads 5,6 must withstand a pull test.

The multilyer capacitor of FIG. 2 has a plurality of layers 10 of green ceramic on each of which is painted an electrode pattern 10a extending to one edge and margined from other edges. In stacking, alternate layers are turned end for end so that in the completed capacitor there are electrodes 11,12 in capacity relation to each other and respectively extending to end terminations 13,14 which are electrically and mechanically connected to leads 15,16 by solder 17,18.

After stacking the green layers the stack is fired to eliminate binders, etc in the green ceramic and vehicle in the paint and to produce a sintered monolith with embedded electrodes.

EXAMPLE I

In Example 1 after the dielectric has been fired, the electrodes 3,4 are applied as patterns of paint of pigment size particles of tin (melting point about 232° C.) and zinc (melting point 419.4° C.) dispersed or suspended in a vehicle of inert or indifferent material which does not react with the pigment particles or the ceramic.

The size of the pigment particles of tin and zinc should be small so that the particles make conductive contact with each other. There is no advantage in particles of diameter smaller than one micron. Particles of diameter greater than 40 microns may not make the desired electrical contact with each other.

In a tin-zinc pigment system, preferred proportions by weight are 65-95% tin and 35-5% zinc. Preferably the tin particles which are the major component of the paint range from one to forty microns in diameter and the zinc particles range from one to twenty microns in diameter. The zinc and tin particles are ball milled together in any suitable inert or indifferent vehicle previously used for silver paint and the paint is applied by silk screening or by any of the other methods previously used for the silver paint. After the application of the electrode patterns, the paint is fired on in air at a temperature in the range of 1000°-1450° F. (539°-788° C.). At temperatures below 1000° F., the particles of tin and zinc have poor contact with the ceramic and high losses and low capacitance are obtained. At temperatures above 1450° F., the conductivity of the electrode is excessively reduced by oxidation of the particles. Within the 1000°-1450° F. range the tin and zinc particles are sintered together forming a new compound of good conductivity and adherence to the ceramic. Prior to the sintering, the characteristic X-ray diffraction spectral lines of tin and zinc can be observed. After the sintering, new spectral lines appear. Also, and unexpectedly, the tin and zinc particles without any frit are bonded to the ceramic as tightly as the prior art frit containing silver paint. This takes place without any adverse reaction between the pigment particles and the ceramic.

The bonding of the Tin-Zinc pigment to the ceramic is surprising. Tin or Zinc particles alone will not bond to the ceramic. Nor will Tin-Lead solders bond to ceramic. Nor will noble metals stick to the ceramic. The combination of Tin and Zinc apparently forms an alloy which either wets the ceramic or has a surface reaction with the ceramic which forms a bond. This is the best theory known to date. The dielectric constant of the ceramic as evidenced by the measured capacitance is substantially unchanged. The insulation resistance is slightly less than the same dielectric with silver paint electrodes from which it might be inferred that there had been some dispersion of the pigment particles into the ceramic. Possibly, the more intimate contact obtained by the bonding of the pigment particles to the ceramic without any frit, and the increase in capacitance which might be expected from the more intimate contact is offset by a slight surface reaction with the ceramic.

The Tin-Zinc electrodes are completely interchangeable with the standard silver electrodes. The variations in properties which appear are within the normal limits of variations for dielectrics with the standard silver electrodes.

The leads may be attached to the electrodes by several methods. First, the electrodes may be copper sprayed and then soldered by solder dip methods. Second, the electrodes may be solder sprayed and the leads attached either by solder reflow or by solder dip. The leads may also be attached by electrically conductive epoxy cement.

The following results were obtained with the Tin-Zinc metallizations:

    __________________________________________________________________________     Dielectric                                                                            K 1200 K1800                                                                               N0470 K300  N2200                                           Capacitance                                                                           3.73nf 4.9nf                                                                               707pf 157pf 858pf                                           D F %  1.23   1.13 .29   1.03  .19                                             IR Gohms                                                                              500    500  1000  500   500                                             at 1000 volts                                                                  RC Ohm 1865   2449 1.07  78.5  429                                             farads                                                                         Successful                                                                            2000VDC                                                                               1750VDC                                                                             2000VDC                                                                              3000VDC                                                                              1750VDC                                         1000 Hrs.                                                                              85° C.                                                                         85° C.                                                                       85° C.                                                                        85° C.                                                                        85° C.                                  life tests                                                                            1500VDC                                                                               1250VDC                                                                             1000VDC                                                                              2000VDC                                                                              750VDC                                          @ 125° C.                                                                      @ 125° C.                                                                      @ 125° C.                                                                    @ 125° C.                                                                     @ 125° C.                                      1000VDC                                                                        @ 150° C.                                                               __________________________________________________________________________

The foregoing tests were made on disc capacitors, the capacitors also passed the industry test requirements for disc capacitors.

EXAMPLE II

Another base metal pigment for metallization which replace silver is nickel (melting point 1452° C.). As obtained, for example from nickel carbonyl, the nickel particles have a surface oxide coating which must be removed and replaced by a coating which prevents oxidation. Oxide removal may be accomplished by treatment in acid solutions. A specific acid treatment consists of washing the powder in a water-nitric acid solution from 5-30% by volume nitric acid for a period of 1 to 5 minutes followed by a distilled water rinse followed by a dilute 1-2% by volume sulfuric acid wash followed by a 30 second rinse in distilled water followed by a hot water rinse. The powder is now submerged immediately in a solution of 10 grams sodium borate plus 10 grams ammonium borate in 100 grams of water, the solution decanted and the powder oven dried. The nickel powder now has an extremely thin coating of nickel borate, possibly mono molecular, an oxidation preventing coating. The powder may now be used as the sole pigment of a metallization paint or paste or the powder may be mixed with other base metal pigments such as tin or zinc. The range of proportions is 99-50% nickel powder to 50-1% base metal.

For tin best results are obtained with the addition of 5% or less (95% nickel 5% tin).

For zinc alone or tin zinc mixture, the addition may be up to 50% by weight. (99-50% nickel 50-1% zinc or zinc tin).

The treatment of the nickel powder to form the oxidation resisting coating is important. If the treatment is omitted, air firing at 1400 degrees F. would result in complete oxidation of the nickel.

The nickel pigment is dispersed in the same vehicle and is applied in the same manner as the silver pigment it replaces. For disc capacitors, the paint is applied after the ceramic is fired. For monolithic or multiple layer capacitors having dielectrics which fire in the range of 800 to 900 degrees C. (1472° to 1652° F.) the nickel pigment paint is applied to the green ceramic and the monolith is fired to produce the monolithic capacitor in one step. Barium titanate and other high K dielectrics which fire in the range of 800 to 900 degrees C. are known. Some of these are glass ceramics--special forms of divitrified glass. Others are glass bonded ceramics consisting of high temperature fired ceramic particles bonded by low temperature glass. Others are based on elements related to titanium.

The nickel metallizations may be soldered directly with an active flux such as Kester 1544 or Kester 815. Alternatively, any of the methods previously disclosed for the tin-zinc metallizations may be used. Also the metallization may be nickel plated or gold plated and then soldered directly.

The coated nickel pigment has surprising adhesion to the ceramic. Nickel powder alone will not adhere. The firing temperature is way below the temperature at which nickel particles sinter. The borate coating may form a glass or it may act as a flux which cleans the surfaces of the nickel pigment particles and promotes a diffusion bonding. The boron in the coating may be a replacement for the usual oxide film which forms on nickel. Another theory is that the nickel borate may form a nickel boride compound which is oxidization resistant and conducting. Whatever the theory, the pigment of coated nickel has excellent adherence to the ceramic and is solderable. Neither of these properties would be obtained if the oxidation resistant coating were omitted.

Examples of capacitors with metallization of Nickel pigment on barium titanate dielectrics are given below:

    __________________________________________________________________________            Pigment                                                                             100% Nickel                                                               Paint                                                                                80% Pigment                                                                          20% Vehicle                                                 __________________________________________________________________________     Dielectric                                                                            K9000                                                                              K5000                                                                              K1200                                                                              K1800                                                                              K3900                                                                              N3300                                                                              N0470                                           Capacitance                                                                           26nf                                                                               15.61nf                                                                            5.74nf                                                                             7.23nf                                                                             4.36nf                                                                             .77nf                                                                              6.47nf                                          DF %   1.77                                                                               1.52                                                                               2.27                                                                               1.45                                                                               .78 3.9 2.1                                             K      9588                                                                               6781                                                                               2506                                                                               2250                                                                               2221                                                                               345 28.8                                            __________________________________________________________________________            Pigment                                                                             100% Nickel                                                               Paint                                                                                60% Pigment                                                                          40% Vehicle                                                 __________________________________________________________________________     Dielectric                                                                            K1700                                                                              K3900                                                                              N0470                                                                              N2200                                                       Capacitance                                                                           4.90nf                                                                             5.26nf                                                                             .144nf                                                                             .39nf                                                       DF %   3.63                                                                               2.45                                                                               1.47                                                                               1.11                                                        K      1936                                                                               2737                                                                               58.4                                                                               64.5                                                        __________________________________________________________________________            Pigment                                                                             100% Nickel                                                               Paint                                                                                60% Pigment                                                                          40% Vehicle                                                             (Metallization Nickel plated                                                   before soldering to leads)                                         __________________________________________________________________________     Dielectric                                                                            K9000                                                                              K5000                                                                              K1700                                                                              K1800                                                                              N3300                                                                              K3900                                               Capacitance                                                                           17.5nf                                                                             13.2nf                                                                             4.76nf                                                                             6.86nf                                                                             5.38nf                                                                             5.58nf                                              DF %   1.9 2.07                                                                               5.25                                                                               1.44                                                                               1.91                                                                               .97                                                 K      7808                                                                               5908                                                                               1881                                                                               2503                                                                               287 2904                                                __________________________________________________________________________            Pigment                                                                             100% Nickel                                                               Paint                                                                                50% Pigment                                                                          50% Vehicle                                                             (Metallization Copper sprayed before                                           soldering leads)                                                   __________________________________________________________________________     Dielectric                                                                            K9000                                                                              K5000                                                                              K1700                                                                              K3900                                                                              N3300                                                                              N0470                                                                              K0300                                           Capacitance                                                                           23.9nf                                                                             12.71nf                                                                            8.31nf                                                                             5.0nf                                                                              6.67nf                                                                             .147nf                                                                             .202nf                                          DF %   2.45                                                                               1.91                                                                               2.45                                                                               1.73                                                                               .29 1.18                                                                               .74                                             K      9200                                                                               5751                                                                               3196                                                                               3167                                                                               299 59.6                                                                               .347                                            __________________________________________________________________________             Pigment                                                                             621/2% Ni                                                                             371/2% Zinc                                                        Paint                                                                               80% Pigment                                                                           20% vehicle                                                __________________________________________________________________________     Dielectric                                                                             K 5000                                                                               K 9000                                                                               N3300 N0470                                                Capacity                                                                               10.8nf                                                                               16.9nf                                                                               589pf 108pf                                                DF %    .59   1.72  .195  .61                                                  IR G ohms                                                                              194   154   765   184                                                  @ 1000 volts                                                                   RC ohn farads                                                                          2095  2603  451   1,98                                                 Successful                                                                             1750VDC                                                                              1000VDC                                                                              1500VDC                                                                              1000VDC                                              life test                                                                              @ 85 C                                                                               @ 85 C                                                                               @ 85 C                                                                               @85 C                                                1000 hour                                                                              1000VDC                                                                              750VDC                                                                               750VDC                                                             @ 125 C                                                                              @ 125 C                                                                              @ 125 C                                                    __________________________________________________________________________            Pigment                                                                             60% Zinc                                                                              40% Nickel                                                         Paint                                                                               80% pigment                                                                           20% Vehicle                                                 __________________________________________________________________________     Dielectric                                                                            K9000                                                                              K5000                                                                              K1200                                                                              K1800                                                                              K3900                                                                              K0300                                                                              N0470                                                                              N2200                                                                              N3300                                   Capacitance                                                                           16.4nf                                                                             10.6nf                                                                             3.99nf                                                                             5.08nf                                                                             4.82nf                                                                             .186nf                                                                             .113nf                                                                             .991nf                                                                             .562nf                                  IF %   1.23                                                                               1.29                                                                               4.48                                                                               2.59                                                                               3.63                                                                               1.87                                                                               .70 1.35                                                                               .30                                     K      5807                                                                               3286                                                                               1576                                                                               2197                                                                               2503                                                                               319 59.3                                                                               162 311                                     __________________________________________________________________________

The approximate composition of dielectrics listed above are:

    __________________________________________________________________________     BaTiO.sub.3                                                                              CaTiO.sub.3                                                                         BiTiO.sub.3                                                                         Minor Constituents                                         __________________________________________________________________________     K 9000                                                                              88%  11%       1% Bismuth, Zinc Oxides                                    K 5000                                                                              87%  12%       1% Fe.sub.2 O.sub.3                                        K 1200                                                                              84%            11% Bismuth Stannate                                                           5% Calcium, Cobalt, Magnesium                              K 1800                                                                              93%       5%   2% Tantalum & Rare Earth                                   K 3900                                                                              70%  25%       5% Rare Earth                                              KO 300                                                                              70%  10%       10% Strontium Titanate                                                         10% Rare Earth                                             N 3300                                                                              70%  30%       Trace Rare Earth                                           NO 470                                                                              99%            1% Rare Earth                                              N 2200                                                                              65   35        Trace Rare Earth                                           K 1700              90% SrTiO.sub.3 plus 10% minor                                                 constituents                                               __________________________________________________________________________ 

What is claimed is:
 1. A capacitor having one or more electrodes of a metallization of finely divided non-noble metals consisting essentially of by weight 65-95 parts tin and 35-5 parts zinc.
 2. A disc capacitor having a sintered ceramic dielectric with electrodes of a metallization of finely divided non-noble metals consisting essentially of by weight 65-95 parts tin and 35-5 parts zinc. 